In the field of working metal products, in particular long or rod-shaped metal products, it is known to provide a final pass in a so-called straightening machine, which has the function of conferring the definitive dimensional tolerances on the products.
It is widely known in the field to use straightening machines equipped with hyperbolic rolls, that is, opposite rolls mounted obliquely with respect to each other, with the purpose of conferring on the metal product in transit between them a roto-translational movement which determines both an advance and a rotation on their axis.
The passage gap defined between the two rolls can generally be adjusted depending on the size of the product being worked.
Generally, known straightening machines also have a safety/control device used to protect the integrity and the functioning of the machine if conditions of overload occur due to the fact that the products entering the machine have excessive sizes with respect to the measurements for which the machine has been set.
Overload conditions occur when a pressure is applied on a roll which is higher than the maximum established; this occurs, for example, if the products are over-sized, or particularly oval shaped, and therefore need a bigger gap than the one between the two rolls.
Known straightening machines can be grouped into two main classes: the first provides that both rolls, both the upper and the lower one, are mobile and can be moved vertically for the initial setting of the correct passage gap for the product to be worked. The second class, on the other hand, provides that only the upper roll is moved vertically, while the lower roll is fixed.
In the case of straightening machines belonging to the first class, the safety/control device is positioned on both the rolls so that, if it is activated, it determines the movement of both rolls. An example of this type of straightening machine is described in DE19724300, in which both rolls have a hydraulic system which serves both to define the initial gap and also to control the overload.
For straightening machines belonging to the second class, the safety/control device is positioned only on the upper roll, as it is the only one which can move.
In this case, the upper roll is not only adjusted mechanically or hydraulically, but also is supplied with a hydraulic system to maintain the force that develops between the roll and the product below a determinate value, beyond which the hydraulic system determines the lifting of the roll.
In known straightening machines, the movement of one or both rolls is obtained by using one or more hydraulic cylinders, which can be either double effect or single effect, or a combination of both.
One disadvantage of known straightening machines with hyperbolic rolls belonging to the first class is that both the movement unit and the control/safety device, positioned on both rolls, are complex to produce and manage.
Another disadvantage shared by straightening machines of both the first and second class is that the movement of the upper roll, if the control/safety device is activated, entails using a great force to lift it (the weight of one of these rolls varies from a few hundred kilograms to some tonnes, depending on the size of the machine). This determines a rather long reaction time, much longer than should be necessary to respond adequately to a situation of potential damage to the machine.
Another disadvantage is that, during maintenance of the control/safety device and of the upper roll, it is necessary to work at some meters from the ground, and also to use movement means such as cranes or gantries, to free the access to the individual parts of the machine; this entails considerable costs and working times.
U.S. Pat. No. 4,763,504 A describes a straightening machine with hyperbolic rolls in which there is an adjustment and safety device against overloads, associated to a lower roll. The device comprises an adjustment screw supported by a hollow piston that is kept in a normal position by a fixed holding element. The hollow piston supports the adjustment screw in correspondence with a surface that yields together with the piston when the pressure applied to the roll exceeds a determinate value.
One purpose of the present invention is to simplify the general construction of the device, its operating management, and all those operations to maintain and replace the parts and to ensure safety for the operators, thus allowing a considerable saving in both time and equipment, and hence entailing a considerable advantage from an economic point of view.
Another purpose of the present invention is to obtain a straightening machine of the type with hyperbolic rolls having limited reaction times when the control/safety device is activated in the event of overloads on the rolls or other operating disadvantage.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.